It has been translated as "vital energy", "life force", or "Breath" but no single English word will suffice.
This is because of the nature of Qi. It can have different manifestations in different situations.
Qi can even be material and is said to "condense" into matter. This is what occurs according to TCM, when a child is conceived and a human being develops. Qi condenses to form a material being. In pathology, when Qi flow slows down and stagnates, masses, lumps or tumors can appear.
The concept of Qi representing a continuum between immaterial and material is not so far from the concepts of matter and energy in modern physics. Matter is not "solid", but its particles are vibrating at various speeds; the faster they vibrate, the more rarefied the matter becomes. In the same way, the manifestations of Qi cover the continuum of matter-energy.
Characters for Qi mean "vapor" or "steam" and "uncooked rice".
This illustrates that Qi can appear in different states ranging from more immaterial like steam to dense and material like rice.
It also illustrates a CENTRAL IDEA in TCM, that the major source of Qi is transformed from the FOOD that we put into our bodies.
Ancient philosophers saw human Qi as the result of the interaction between the Qi of Heaven (Yang) and Earth (Yin). They stressed the interaction between a human being's Qi and the forces of nature.
TCM, following these ancient philosophies, stresses the relationship between human beings and natural forces, both cosmic and local (the environment).
Example: When we say Spleen Qi, we mean the complex of the functional activities of the Spleen.
Example: Defensive Qi circulates primarily in the Exterior to protect the body. Nutritive Qi circulates in the Interior, to nourish the Organs. These two are different manifestations of Qi. When either is unable to perform its functions, specific pathological symptoms will result.
The various forms of Qi are discussed below. However, it is first important to understand another vital substance: Jing.